The problem of bacterial resistance to antibacterial agents has been threatening humans over many years. Examples of such difficult-to-treat resistant bacteria include methicillin-resistant Staphylococcus aureus (MRSA), penicillin-resistant Streptococcus pneumoniae (PRSP), and vancomycin-resistant Enterococcus (VRE). As existing antibacterial agents such as β-lactam, quinolone, and macrolide agents are used clinically, the number of bacteria resistant thereto goes on increasing. On the other hand, the total number of novel antibacterial agents launched has been decreasing since the late 1980s. The problem of bacterial resistance to antibacterial agents is considered as a more critical issue than ever before (Nathan, C. 2004, Nature 431: 899-902). For overcoming the threat of an increasing number of resistant bacteria, there is a need to develop novel antibacterial agents, particularly, antibacterial agents having a novel mechanism of action effective even for various resistant bacteria.
Among bacterial infectious diseases, community-acquired respiratory infection occurs with the highest frequency, which is mainly caused by bacteria including a gram-positive bacterium Streptococcus pneumoniae and a gram-negative bacterium Haemophilus influenzae. Gram-positive bacteria have only a cytoplasmic membrane consisting of a phospholipid bilayer, whereas gram-negative bacteria have, in addition to a cytoplasmic membrane (inner membrane), an outer membrane consisting of an asymmetric bilayer of lipopolysaccharide and phospholipid and therefore exhibit resistance to many antibacterial agents. Only some antibacterial agents used clinically exhibit effectiveness for both Streptococcus pneumoniae and Haemophilus influenzae. Most novel drugs launched from the 21st century and novel drug candidates under clinical tests are effective only for the gram-positive bacterium and are inferior in effectiveness for Haemophilus influenzae. There is a demand to develop a novel antibacterial agent that is effective for both Streptococcus pneumoniae and Haemophilus influenzae and is applicable to community-acquired respiratory infection.
Deoxyribonucleic acid (DNA) gyrase is one kind of type II topoisomerase which regulates the topological state of intracellular DNA (Champoux, J. J., 2001, Ann. Rev. Biochem. 70: 369-413). The type II topoisomerase modifies DNA topology by catalyzing a series of reactions through which a DNA duplex is transiently nicked by use of free energy generated as a result of hydrolysis of adenosine triphosphate (ATP); a strand is passed through the nick; and the nicked DNA is reannealed. The DNA gyrase regulates the supercoiling of DNA and relieves topological stress caused by the unwinding of a parent DNA duplex during the replication process. The DNA gyrase is an enzyme essential for growth to be conserved in bacteria and is characterized, among topoisomerases, in the ability to introduce a negative supercoil to DNA.
The DNA gyrase is a protein tetramer consisting of two A subunits (GyrA) and two B subunits (GyrB). GyrB consists of an amino-terminal domain having ATP hydrolytic activity and a carboxy-terminal domain which interacts with GyrA and DNA. The supercoiling reaction is initiated by ATP binding to GyrB. Subsequently, the ATP is hydrolyzed during this reaction. This ATP binding and the subsequent hydrolysis cause a change in the higher-order structure of the DNA-bound gyrase. This is essential for the activity of DNA gyrase.
In contrast to DNA gyrase, eukaryotic type II topoisomerase is a homodimer that is capable of relaxing negative and positive supercoils but not capable of introducing a negative supercoil. Ideally, an antibacterial agent based on the inhibition of bacterial DNA gyrase is selective for this enzyme and has no or relatively weak inhibitory activity against eukaryotic type II topoisomerase.
Another bacterial type II topoisomerase is referred to as topoisomerase IV, which is mainly involved in the segregation of catenated closed circular chromosomes produced during replication. The topoisomerase IV is a protein tetramer consisting of two ParC subunits and two ParE subunits. ParE consists of an amino-terminal domain having ATP hydrolytic activity and a carboxy-terminal domain which interacts with ParC. These subunits ParC and ParE are highly homologous to GyrA and GyrB, respectively. The hydrolysis of ATP is required for getting the enzyme back into the initial state and restarting catalytic cycles. TopoIV is highly conserved in bacteria and is essential for bacterial replication (Drlica, K. and Zhao, X., 1997, Microbiol. Mol. Biol. Rev. 61: 377-392).
DNA gyrase is targeted by quinolone antibacterial agents. Quinolone binds to GyrA to form a tripartite complex of quinolone, DNA gyrase, and DNA. This agent induces cell death by inhibiting DNA replication in this way. Moreover, some members in this class of antibacterial agents also inhibit topoisomerase IV. As a result, these compounds differ in their primary targets among bacterial species and among the compounds. Although the quinolone agents are effective antibacterial agents, they increasingly raise the problem of resistance caused by the mutation of the targets (DNA gyrase and topoisomerase IV) in several kinds of organisms such as Staphylococcus aureus and Streptococcus pneumoniae (Hooper, D. C., 2002, Lancet Infectious Diseases 2: 530-538). In addition, the quinolone antibacterial agents have been confirmed to have adverse reactions (joint or cartilage disorders) in immature animals and are thus prevented from being used in children (Lipsky, B. A. and Baker, C. A., 1999, Clin. Infect. Dis. 28: 352-364). Examples of other adverse reactions of the quinolone antibacterial agents include cardiotoxicity (prolonged QT interval), decreased blood sugar levels, photosensitivity, and convulsion caused by combined use with a nonsteroidal anti-inflammatory agent.
Many conventional quinolones inhibit topoisomerase IV more strongly than DNA gyrase in many gram-positive bacteria and inhibit DNA gyrase more strongly than topoisomerase IV in many gram-negative bacteria. Some novel quinolones exhibit more equally balanced inhibitory activity against DNA gyrase and topoisomerase IV in one bacterial species. More highly sensitive intracellular (primary target) enzymes become resistant to quinolones as a result of a point mutation. However, a quinolone that equally inhibits DNA gyrase and topoisomerase IV inhibits a secondary target enzyme even if its primary target enzyme is mutated; thus the resistance level is low and limited (Hooper, D. C., 2000, Clin Infect Dis. 31: S24-S28). DNA gyrase and topoisomerase IV are highly homologous in their amino acid sequences. Compounds targeting bacterial type II topoisomerase have the potential to inhibit both these two targets in the cell.
There exist several kinds of known natural products that compete with ATP for binding to GyrB and inhibit DNA gyrase (Maxwell, A. and Lawson, D. M., 2003, Curr. Topics in Med. Chem. 3: 283-303). Coumarin antibacterial agents are natural products isolated from the genus Streptomyces. Examples thereof include novobiocin, clorobiocin, and coumermycin A1. Although these compounds are potent inhibitors for DNA gyrase, they have toxicity to eukaryotes and low permeability into the bodies of gram-negative bacteria and therefore have low efficacy in clinical application (Maxwell, A., 1997, Trends Microbiol. 5: 102-109). Examples of other natural products targeting GyrB include cyclothialidine isolated from Streptomyces filipinensis (Watanabe, J. et al., 1994, J. Antibiot. 47: 32-36) and cinodine isolated from the genus Nocardia (Martin, J. H. et al., 1978, J. Antibiot. 31: 398-404). Cyclothialidine is an inadequate antibacterial agent that exhibits activity against only a limited number of bacteria (Nakada, N. et al., 1993, Antimicrob. Agents Chemother. 37: 2656-2661). Cinodine has strong toxicity to eukaryotes and is therefore impossible to use clinically (Ellestad, G. A., 2006, Journal of Medicinal Chemistry. 49: 6627-6634). There is a demand to acquire a novel effective GyrB inhibitor that overcomes the disadvantages of these known natural products. Such an inhibitor is an interesting antibacterial agent candidate that is effective even against the spread of resistant bacteria, which besets the existing antibacterial agents.
In developing antibiotics having a novel mechanism of action, synthetic inhibitors targeting the DNA gyrase GyrB subunit are known in the art. Patent Publication Nos. WO 2005/026149, WO 2006/087543, WO 2006/087544, WO 2006/087548, WO 2006/092599, and WO 2006/092608 describe pyrrole derivatives having antibacterial activity. Patent Publication No. WO 2007/071965 describes bicyclic heteroaromatic compounds. These compounds had the problems of insufficient activity, cytotoxicity, low solubility in water, and the production of electrophilic reactive metabolites.    Patent Document 1: WO 2005/026149    Patent Document 2: WO 2006/087543    Patent Document 3: WO 2006/087544    Patent Document 4: WO 2006/087548    Patent Document 5: WO 2006/092599    Patent Document 6: WO 2006/092608    Patent Document 7: WO 2007/071965